Abstract: One method includes epitaxially growing a layer of group III-nitride semiconductor under growth conditions that cause a growth surface to be rough. The method also includes performing an epitaxial growth of a second layer of group III-nitride semiconductor on the first layer under growth conditions that cause the growth surface to become smooth. The two-step growth produces a lower density of threading defects. Another method includes epitaxially growing a layer of group III-nitride semiconductor on a lattice-mismatched crystalline substrate and then, chemically treating a growth surface of the layer to selectively electrically passivate defects that thread the layer.